Instructables member pranksgiving has a great/terrible idea for a prank and instructions on how to carry it out. You can follow them to create the impression that someone left a bundled dead human body by the dumpster or your co-worker's cubicle.
He offers this disclaimer:
It's only garbage, but there's a very real risk of this prank being taken seriously. Don't recreate this prank. Do not leave garbage body unattended, and always let inquisitive folk in on the joke.
But won't that spoil all of the fun? Besides, how will you get in serious trouble if you let people in on the prank?